The present invention relates generally to an anti-lock or anti-skid brake control system for an automotive vehicle for controlling a hydraulic brake system of the vehicle to optimize braking characteristics. More specifically, the invention relates to a method and system for recording the input timing of wheel speed sensor signal pulses, in which overflow of a timer which measures the intervals between the wheel speed sensor signal pulses is detected so that a back-up operation can be performed.
As is well known, in anti-skid control, the braking force applied to wheel cylinders is so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. 80%, of the vehicle speed. Such a practice has been believed to be effective, especially when road conditions and other factors are taken into consideration. Throughout the accompanying disclosure, the ratio of wheel peripheral speed to vehicle speed will be referred to as "slip rate" or "slip ratio".
U.S. Pat. No. 4,267,575, issued on May 12, 1981 to Peter BOUNDS, discloses a system, which serves to provide signals to a microcomputer-based control system from which instantaneous values of speed can be computed, includes a wheel-driven alternator which provides an alternating current output whose frequency varies with wheel speed. A signal processor converts this signal to a series of sensor pulses whose width varies inversely with frequency. A sample pulse supplied by a microprocesor sets the period or length of time during which the sensor pulses are examined for each speed calculation cycle of the microprocessor. The sample period pulses are AND-gated with a high-frequency clock signal and also with the sensor pulses to provide a series of marker pulses marking the up and down excursions of the sensor pulses. The marker pulses occurring in each sample period are counted directly in a first counter, and in addition are supplied to a latch circuit and from thence to an AND gate which responds to the first marker pulse in the sample period to count occurrences of the first counter exceeding its capacity. A third counter is also connected to receive the high-frequency clock pulses and counts only the clock pulses occurring after the last marker pulse in the sample period. At the end of the sample period, the counts from all three counters are transferred to the microprocessor which uses this information to compute a value for wheel velocity over the sample period. The system continuously provides the input counts to enable the microprocessor to calculate wheel velocity over each sample period.
In addition, U.S. Pat. No. 4,315,213, issued on Feb. 9, 1982 to Manfred WOLFF, discloses a method for obtaining an acceleration or deceleration signal from a signal proportional to speed and apparatus therefor. The method for obtaining an acceleration or deceleration signal from a signal proportional to the speed consists of storing the n most recently ascertained changes in the speed signal in a memory, and upon ascertainment of a new change to be stored in memory, erasing the change which has been stored the longest, and forming a deceleration or acceleration signal by addition of the stored n changes periodically at intervals of dT. In this method, the occurrence of deceleration or acceleration exceeding the threshold is recognized quickly.
In another approach, U.S. Pat. No. 4,384,330 to Toshiro MATSUDA, issued on May 17, 1983 discloses a brake control system for controlling application and release of brake pressure in order to prevent the vehicle from skidding. The system includes a sensing circuit for determining wheel rotation speed, a deceleration detecting circuit for determining the deceleration rate of the wheel and generating a signal when the determined deceleration rate becomes equal to or greater than a predetermined value, a target wheel speed circuit for determining a target wheel speed based on the wheel rotation speed and operative in response to detection of a peak in the coefficient of friction between the vehicle wheel and the road surface, and a control circuit for controlling application and release of brake fluid pressure to wheel cylinders for controlling the wheel deceleration rate. The wheel rotation speed sensing circuit detects the angular velocity of the wheel to produce alternating current sensor signal having a frequency corresponding to the wheel rotation speed. The wheel rotation speed sensor signal value is differentiated to derive the deceleration rate.
Another approach for deriving acceleration has been disclosed in U.S. Pat. No. 3,943,345 issued on Mar. 9, 1976 to Noriyoshi ANDO et al. The system disclosed includes a first counter for counting the number of pulse signals corresponding to the rotational speed of a rotating body, a second counter for counting the number of pulses after the first counter stops counting, and a control circuit for generating an output signal corresponding to the difference between the counts of the first and second counters.
In the present invention, another approach has been taken to derive the wheel rotation speed which will be hereafter referred to as "wheel speed" based on input time data representative of the times at which wheel speed sensor signal pulses are produced. For instance, by latching a timer signal value in response to the leading edge of each sensor signal pulse, the intervals between occurrences of the sensor signal pulses can be measured. The intervals between occurrences of the sensor signal pulses are inversely proportional to the rotation speed of the wheel. Therefore, wheel speed can be derived by finding the reciprocal of the measured intervals. In addition, wheel acceleration and deceleration can be obtained by comparing successive intervals and dividing the obtained difference between intervals by the period of time over which the sensor signals were sampled.
To perform this procedure, it is essential to record the input timing in response to every sensor signal pulse. A difficulty is encountered due to significant variations in the sensor signal intervals according to significant variations in the vehicle speed. In recent years, modern vehicles can be driven at speeds in the range of about 0 km to 300 km. Sensor signal intervals vary in accordance with this wide speed range. In particular, when the vehicle is moving at a relatively high speed, the input intervals of the sensor signal pulses may be too short for the anti-skid control system to resolve. As accurate sampling of input timing is essential for the proposed approach, errors in the recorded input time data will cause errors or malfunction of the anti-skid brake control system. One possible source of error in sampling the input timing is accidentally missing one or more sensor signal pulses. Such errors are particularly likely to occur when the vehicle and wheel speeds are relatively high and therefore the intervals between adjacent sensor signal pulses are quite short.
U.S. Pat. No. 4,408,290, issued on Oct. 4, 1983 to the common inventor of this invention is intended to perform the foregoing input time data sampling for use in calculation of acceleration and deceleration. In the disclosure of the applicant's prior invention, an acceleration sensor acts on the variable-frequency pulses of a speed sensor signal to recognize any variation of the pulse period thereof and to produce an output indicative of the magnitude of the detected variation to within a fixed degree of accuracy. The durations of groups of pulses are held to within a fixed range by adjusting the number of pulses in each group. The duration of groups of pulses are measured with reference to a fixed-frequency clock pulse signal and the measurement periods of successive groups of equal numbers of pulses are compared. If the difference between pulse group periods is zero or less than a predetermined value, the number of pulses in each group is increased in order to increase the total number of clock pulses during the measurement interval. The number of pulses per group is increased until the difference between measured periods exceeds the predetermined value or until the number of pulses per group reaches a predetermined maximum. Acceleration data calculation and memory control procedure are designed to take into account the variation of the number of pulse per group.
The applicant's prior invention is effective for expanding intervals for sampling the input time data of the sensor pulse signals and for enabling the anti-skid control system to resolve variations in the wheel speeds. However, when the rotational speed of wheel drops to a relatively low speed, the concominantly longer intervals between the wheel speed sensor signal pulses pose a problem. Specifically, a clock counter associated with a clock generator and adapted to measure the period of time between the wheel speed sensor signal pulses or groups thereof will have a limited counting capacity. Thus, when the pulse input interval is excessively long, overflow of the counting capacity within one cycle of wheel speed measurement may occur. If overflow of the clock counter occurs and is ignored, the control factors such as wheel speed derived from the pulse input timing data will be highly inaccurate, resulting in faulty skid control operation.